It is known to use rotating lasers on building sites, for example of buildings or in roadbuilding work and/or groundworks. In particular, rotating lasers are used in which a laser beam (in the visible or infrared wavelength range) emitted by a laser unit generates a reference area, by deflection via a rotating deflecting prism, by means of which reference area a precise plane reference (in particular a height reference in the case of a horizontal plane) is then provided.
Many of the rotating lasers in existence nowadays have a beam self-leveling functionality. In order to fulfill such a beam self-leveling functionality, various technical solutions are known which can be purely mechanical in nature but are nowadays usually based on a sensor system which is optical in nature. For example, the core of the rotating laser (i.e. the laser core module), which comprises in particular the laser unit and the rotatable deflecting prism, can be suspended in oscillating fashion, with the result that leveling accuracy can be produced using gravity. However, in this case, the laser core module can advantageously be suspended on an outer housing of the device in a manner such that it can be inclined precisely, in motorized fashion, about two axes (at least slightly in a range of, for example, ±5°) and can be equipped with an inclination sensor or leveling sensor, whose indication or signal can be read and used as output variable for actively changing the position of inclination of the laser core module.
Depending on the development stage, known rotating lasers nowadays in this case also have a function (with corresponding mechanical system, sensor system and control system) for the dedicated, desired inclination of the laser plane relative to the horizontal in one or two directions. For this purpose, the core of the rotating laser, which in particular comprises the laser unit and the rotatable deflecting prism, can be inclined in a targeted manner in motorized fashion about an axis or about two axes and brought into desired positions of inclination, with the result that, therefore, the axis of rotation and consequently also the plane spanned are also inclined in a desired manner. Corresponding mechanisms, sensor systems and control systems for this have long been known from the prior art and are described, for example, in the patent literature publications U.S. Pat. No. 5,485,266 A, US 2004/0125356 A1, EP 1 790 940 A2, EP 1 901 034 A2, EP 2 327 958 A1 and EP 2 522 954 A1.
If, in this case, the rotating laser beam emitted by the rotating laser is transmitted in the visible spectrum and impinges on an area such as, for example, a wall, a floor or a ceiling of the building, a reference line is visible there as the basis for further measurements.
For precise transmission of the reference plane or reference height given by the rotating laser beam onto a wall or onto the site for example, handheld laser receivers are known which can determine and indicate with high precision a position relative to a reference area spanned by the rotating laser.
Handheld laser receivers known from the prior art for determining a position relative to the reference area can in this case have a laser beam detector, which comprises a multiplicity of photosensitive elements and is designed to generate an output signal in the event of impingement of the laser beam on the laser beam detector. In detail, in this case the laser beam detector is usually designed in such a way that, in addition, an impingement position of the laser beam on the laser beam detector area can be derived, for which purpose the photosensitive elements, when viewed in an upright operating position of the device, can be arranged next to one another in a row in a vertically aligned sensor row, with the result that, therefore, the laser beam detector extends at least over a one-dimensional region on the laser receiver. In addition, usually an evaluation unit for determining the position of the laser receiver relative to the reference height defined by the rotating laser beam on the basis of the output of the laser beam detector and an indicator for the determined position (for example a visual display), in particular designed for indicating whether the laser receiver precisely coincides with the reference area, are integrated in the laser receiver device. In this case, the position can be determined, for example, on the basis of a ratio of a plurality of output signals (for example as the center point of that subregion on the laser beam detector row which is illuminated by the laser beam).
Such handheld laser receivers can be used in particular when the line depicted by the rotating laser beam can be perceived by the eye only with difficulty or not precisely enough. This is the case, for example, at relatively long distances from the rotating laser (for example owing to a divergence of the laser beam [→depicted line is too wide] or a low luminous efficacy [→depicted line is not visible enough] (which is subject to certain limits for eye safety reasons) and/or a high level of ambient brightness) or else when using laser light in the nonvisible wavelength range.
In such cases, it is now possible by means of such laser receivers to find the laser beam and indicate the laser plane (or reference height) defined by a rotating laser beam, read this laser plane and transmit the height information onto the site or onto a wall (etc.). For example, indicated by the laser receiver, a corresponding marking can be applied at the reference height.
For this, the laser receiver is moved by a user searching up and down in the vertical direction, for example, and finally brought into that position in which the indicator indicates a coincidence with the reference area. For example, a visual display which (for example by means of illuminated arrows or differently colored LEDs) provides information on whether a defined zero point of the laser receiver (for example an area center point of the detector area) is located                precisely at the height of the reference area,        above the reference area or        below the reference areacan be provided as indicator.        
Furthermore, a numeric display of the relative position of the laser receiver with respect to the reference height can be used as indication, for example given in mm or inches.
Examples of such laser receivers are disclosed in the documents EP 2 199 739 A1 and U.S. Pat. No. 4,240,208.
In order to provide the user with simple transmission of the reference height determined and indicated by the laser receiver, a height mark can be provided on the housing of the laser receiver at the height of the defined zero point (for example a notch or a printed line laterally on the housing).
For a series of known functions and applications of a system comprising a rotating laser (in particular a dual-grade rotating laser) and a laser receiver, in addition (sometimes at least rough) knowledge of a laser receiver direction may be required or at least helpful, i.e. knowledge of a direction in which the laser receiver is located from the point of view of the rotating laser (for example with respect to a coordinate system which is internal to the rotating laser).
Examples of such functions and applications can in this case be grade-catch (also referred to as plane-catch or slope-catch), grade-lock (also referred to as plane-lock or slope-lock, possibly with tracking) or axis-alignment/axis-finding, as are known to a person skilled in the art. Specific aspects and embodiments with respect to these functions are also described, for example, in the patent literature publications U.S. Pat. No. 6,055,046 A, U.S. Pat. No. 6,314,650 B1 and U.S. Pat. No. 6,693,706 B2.
The following methods are in this case known from the prior art, for example (inter alia also from the publications mentioned in the directly preceding paragraph) for the determination of a laser receiver direction in a system comprising a rotating laser and a laser receiver:
1) Evaluation of a signal generated directly (in real time) after detection by the receiver of a beam, which signal is transmitted from the receiver to the rotator (for example by radio), and derivation of an emission angle at which the rotating laser beam was precisely at the time of impingement.
2) Defined inclination of the reference plane through a known inclination value and reading by the laser receiver of a height offset, effected thereby, of the beam strike on the detector of the laser receiver (with implementation of these steps for both inclination axes) and derivation of a direction to the receiver on the basis of the given relationship between the respective inclination angle difference and the respective height offset on the receiver.
3) Supplying of an information item which varies continuously in a manner dependent on the angle to a beam parameter of the laser radiation, which information item can be read by the receiver on the basis of the impinging beam and can be used to make the direction to the receiver derivable.
4) Iteratively halving windowing following striking or non-striking of the laser receiver in the respectively present angular range window (for example transmission of the beam only in the angular range of 0-180° if the receiver has indicated a strike: transmission of the beam only in the angular range of 0-90° if the receiver has not indicated a strike at 0-180°: transmission of the beam only in the angular range of 180°-270°, etc.).
The topic relating to determination of the laser receiver direction is handled inter alia also in the patent literature publication WO 2006/070009 A2.
However, the invention now relates in particular to the previously already explained beam leveling functionality of a rotating laser, in which the laser core module is suspended on an outer housing of the device such that it can be inclined precisely, in motorized fashion, for example about two axes (at least slightly in a range of, for example, ±5°) and is equipped with one or two inclination sensors or leveling sensors, whose output can be used as the output variable for an active change in the tilt position of the laser core module.
For the beam leveling functionality, adjustment and calibration is performed in the factory, in which such calibration data with respect to interaction of the leveling sensor and the tilting mechanism are stored in a memory that, via the calibration data, depending on an output of the leveling sensor, the mechanism can be actuated in a defined manner and thus the axis of rotation can be tilted in a targeted manner in such a way that the rotating laser beam also actually as precisely as possible spans a horizontal plane.
The adjustment of the laser beam (for example owing to the leveling sensor or the tilting mechanism) can change, however, as a result of various external influences, such as, for example, temperature and moisture fluctuations or mechanical vibrations, etc. Therefore, it is desirable to check and possibly recalibrate the plane or leveling accuracy of the rotating laser beam and its beam self-leveling functionality at regular intervals or as required.
For a recalibration of the rotating laser, in this case a wide variety of methods are known which can always only be implemented purely manually and are often selected and defined individually by a user depending on personal preference or personal knowledge, skills and capabilities.
In addition, for recalibration of the rotating laser, special calibrating telescopes are known, such as the one described in, for example, the European Patent Application with the number EP 12195754.2, which, in practice, is often only used for recalibrations in the factory owing to the complexity associated therewith, however.